The present invention relates to chemicals and a method for imparting a hydrophilic property to aluminum or aluminum alloy substrates such as aluminum fins for heat exchangers, and more particularly to chemicals and a method for providing aluminum or aluminum alloy substrates with lubricating and hydrophilic properties.
Since aluminum and its alloys are light in weight and have excellent workability and heat conductivity, they are widely used for heat exchangers, etc. With air conditioning systems finding wider applications, air conditioners for cooling, warming, dehumidifying, etc., are increasingly used. Heat exchangers of these air conditioners are generally equipped with aluminum alloy fins.
However, it is observed that moisture tends to be condensed and deposited as water droplets on the fin surfaces of air conditioners during cooling operations. If the fin surface is water-repellent, this condensed water tends to be deposited in a hemispherical form on the fin surface or forms bridges between the fins, preventing smooth air flow, which in-turn increases resistance of air flow, thereby decreasing heat exchange efficiency. Accordingly, the fin surfaces are treated so that they have a hydrophilic property.
In the case of working the aluminum or aluminum alloy substrates such as aluminum plates, a working oil is generally applied to aluminum plates precoated with hydrophilic layers for the purpose of improving workability and protecting the surfaces of the aluminum plates. After working, the working oil conventionally is removed in a degreasing step by using a fluorocarbon solvent (Flon) as a detergent. However, the recent trend of environmental protection regulates the use of Flon, so that other working methods without using Flon are being sought. As a result, volatile solvent-type lubricants based on low-boiling point paraffins or naphthenes which do not require a washing step have been being used as working oils for fin plates.
However, when such volatile lubricating oils are used, substrate surfaces have a poor lubricating property, causing such problems as poor workability in stamping, drawing, etc. Thus, it has been desired to provide the hydrophilic coats themselves with a lubricating property to improve their workability.
In these circumstances, Japanese Patent Laid-Open No. 61-183394 discloses a lubricating agent containing fine pulverous water-swellable hydroxypropyl cellulose having a low degree of substitution for cold plastic working, which lubricating agent consists of 15-35 weight % of at least one inorganic lubricant selected from MoS.sub.2 and graphite, 1-5 weight % of at least one water-soluble resin selected from hydroxypropyl cellulose, hydroxypropyl methyl cellulose, and methyl cellulose, and water as primary components. A suitable amount of the water-soluble hydroxypropyl cellulose having a low degree of substitution (preferred degree of substitution is 7-14 weight %, and the substituent is a hydroxypropoxy group) to be added to the composition is 0.1-5.0 weight %.
However, since this lubricating agent contains hydroxypropyl cellulose and/or hydroxypropyl methyl cellulose and/or methyl cellulose and is directly applied to a metal substrate without a thermoset primer coat, the resulting hydrophilic coat is likely to peel off from the metal substrate. Although the resulting hydrophilic layer has a lubricating property, the coating layer is poor in durability.
Japanese Patent-Laid-Open No. 63-170492 discloses a method of working an aluminum fin plate for a heat exchanger in which fine powder of lubricant, such as stearic acid or a fluorine resin, is adhered to the surface of an aluminum fin material in the form of a thin film with a hydrophilic surface, with a water-soluble resin as a binder. Then press working is carried out. The adhesion is carried out at 100.degree.-300.degree. C. by baking. The water-soluble resin is preferably an acrylic acrylamide copolymer.
However, since this layer is removed after working, the resulting fins do not have any hydrophilic property and alkali resistance.
In general, to improve the workability of the metal substrates such as aluminum substrates, such metal substrates are coated with lubricating resin layers, and when they have resin coatings like precoated plates, a lubricating agent is added to the precoat layers to provide them with a lubricating property. For instance, Japanese Patent Publication No. 62-41457 discloses an aluminum plate having excellent drawing workability, a surface of which aluminum plate is coated with a particular amount of a surface treatment solution containing a water-soluble thermoplastic polymer resin consisting of an ammonium salt of a copolymer of .alpha.-olefin and .alpha.,.beta.-unsaturated carboxylic acid. Also, Japanese Patent Laid-Open No. 63-86764 discloses a composition for forming a lubricating layer on a metal surface, which comprises 20-95 weight % of a thermosetting resin and 5-80 weight % of a inorganic lubricant.
However, the surface treatment solution used in Japanese Patent Publication No. 62-41467 and the composition disclosed by Japanese Patent Laid-Open No. 63-86764 fail to produce coating layers having such a good hydrophilic property as to satisfy the requirements of cross fin plates for heat exchangers, etc.
On the other hand, Japanese Patent Laid-Open No. 61-60766 discloses an aqueous composition for forming a lubricating coating layer consisting essentially of a water-soluble or water-dispersible organic resin, an organic-inorganic composite reaction product consisting of an alkoxy- (or alkoxyalkoxy-) silane compound and silica, and a lubricant. The coating layer obtained by this aqueous composition has a hydrophilic property to some extent, but this hydrophilic property is not necessarily sufficient for the present demand. Also, when this composition is applied to cross fins of heat exchangers, it generates an odor peculiar to silica, causing an unpleasant feeling.
As described above, it is difficult to form a coating layer having not only a good hydrophilic property but also an excellent lubricating property, and particularly now that a volatile solvent-type lubricant is required to be used as a working oil due to the prohibition of Flon, it is strongly desired to provide coating layers satisfying the above two requirements.